In a conventional monitoring system, a sensor such as a fire hazard sensor and gas sensor is connected from a receiver device to a transmission line extending to monitor for abnormalities such as a gas leakage (for example, refer to Patent Documents 1 and 2). In this monitoring system, a digital signal, which is a downstream signal such as control information, is transmitted in a voltage mode from the receiver device to a terminal. Meanwhile, the terminal transmits a digital signal, which is an upstream signal such as sensor information, in a current mode to the receiver device.
FIG. 6 shows the conventional monitoring system. As shown in the diagram, transmission lines 102a and 102b serving also as power supply lines are led out from a receiver device 100 serving as a master device, and an analog type sensor 104 and a relay device 106 serving as slave devices are connected thereto. To the analog type sensor 104 and the relay device 106, there are respectively set a unique address.
The analog type sensor 104 detects a concentration of smoke caused by a fire hazard occurrence or an analog value of the surrounding temperature, and transmits smoke concentration data or temperature data to the receiver device 100. The receiver device 100 determines the presence or absence of a fire hazard occurrence based on the smoke concentration data or the temperature data, and issues a fire hazard warning if a fire hazard occurrence is determined.
Sensor lines 108a and 108b are led out from the relay device 106, and a plurality of ON/OFF type sensors 110 which do not have a transmitting function, are connected thereto as loads. When the ON/OFF type sensor 110 detects an indication of a fire hazard occurrence, it allows an alarm current to flow to the relay device 106 via the sensor lines 108a and 108b. When the relay device 106 receives this alarm current, fire hazard alarm data is transmitted from the relay device 106 to the receiver device 100. Then, the receiver device 100 issues a fire hazard warning.
The receiver device 100 sequentially specifies a slave device address, and transmits a polling downstream signal to respective slave devices (the analog type sensor 104 and the relay device 106) in a voltage mode. The slave device which has received this polling downstream signal distinguishes its own address, and returns a transmission current serving as an upstream signal which indicates a normal state, to the receiver device 100.
FIG. 7 is a diagram which shows, with an equivalent circuit, the receiver device 100, the analog type sensor 104, and the relay device 106 in the conventional system shown in FIG. 6. The relay device 106 supplies electric power to the ON/OFF type sensors 110 connected thereto as a load, to thereby steadily supply operating current, and therefore the ON/OFF type sensors 110 can be treated as the load 122 illustrated as a resistor. Accordingly, load current Iz from the load 122 steadily flows to the transmission lines 102a and 102b. 
The analog type sensor 104 is provided with a constant current source 112 and a switch 114. In the analog type sensor 104, for example, with respect to the polling from the receiver device 100, a CPU 116 returns an upstream signal indicating normality to the receiver device 100 with a current pulse signal of a predetermined bit length.
The current pulse signal transmitted from the analog type sensor 104 is input to a transmission input circuit 118 of the receiver device 100, and a current detection voltage pulse signal proportional to this current pulse signal is generated and transmitted to a CPU 120. As a result, the CPU 120 which has read the current detection voltage pulse signal recognizes the analog type sensor 104 as being normal. That is to say, in a state where the load 122 is flowing the load current Iz to the transmission lines 102a and 102b serving also as power supply lines, the transmission input circuit 118 detects the presence or absence of transmission current from the analog type sensor 104 serving as a slave device.
FIG. 8 is a circuit diagram of the conventional transmission input circuit 118 provided in the receiver device 100 shown in FIG. 7. In FIG. 8, in the transmission input circuit 118, a predetermined power supply voltage Vc is applied to the transmission line 102a, while the signal line 102b side is connected to a current detection resistor R11 via a diode D11.
As shown in FIG. 7, the relay device 106 and the analog type sensor 104 are connected to the transmission lines 102a and 102b, and a load current Iz dependant on the load 122 of the relay device 106 is flowed at an empty timing where no transmission current is flowing. When the analog type sensor 104 outputs a transmission signal, a transmission current Ia with the load current Iz added thereto flows.
A detection voltage according to the line current which is produced at both ends of the current detection resistor R11 shown in FIG. 8 is applied to the negative input terminal of a comparator 122. A capacitor C11 is connected to the positive input terminal of the comparator 122, and the capacitor C11 is further connected to the input side of the diode D11 via a switch SW11.
The switch SW11 is switched by the CPU 120 at an empty timing where transmission current Ia from the slave devices such as the analogy type sensor 104 is not flowing, and it sample-holds in the capacitor C11, a reference voltage Vr in which a threshold voltage Vf serving as a forward drop voltage of the diode D11 is added to a load current detection voltage Vz of the current detection resistor R11, that is, Vr=(Va+Vf).
FIG. 9 is a time chart showing a signal waveform of each section in FIG. 8. FIG. 9 (A) shows the input voltage of the comparator 122, and FIG. 9 (B) shows the timing of sampling of the capacitor C11 performed by the switch SW11.
As shown in FIG. 9 (A), a load current detection voltage Vz due to a load current Iz flowing through the transmission lines 102a and 102b in a state where transmission current Ia is not present, is input as a base voltage. Moreover, with switching of the switch SW1 at an empty timing with no transmission current Ia, a reference voltage Vr, in which the threshold voltage Vf serving as the forward drop voltage of the diode D11 is added to the load current detection voltage Vz of the current detection resistor R11, is sample-held in the capacitor C11.
When a transmission current Ia flows due to transmission of a transmission signal from a slave device, at the current detection resistor R11, there is produced a transmission current detection voltage Va corresponding to the transmission current Ia, having the load current detection voltage Vz added thereto. The comparator 122 extracts a reception voltage component (voltage pulse component) which exceeds the reference voltage Vr=(Vz+Vf) held in the capacitor C11, and inputs this as a transmission current detection signal to the CPU 120 which then performs a fire hazard warning process or the like.
FIG. 10 is a time chart shown with the time axis of FIG. 9 contracted. Pulse signals are transmitted from the slave device side at a constant cycle, and at an empty timing thereof, a reference voltage Vr=(Vz+Vf), in which a threshold voltage Vf serving as a forward drop voltage of the diode D11 is added to a load current detection voltage Vz, is sample-held in the capacitor C11. Then, a voltage component which exceeds the reference voltage Vr of a transmission current detection voltage Va obtained immediately thereafter is detected, and input to the CPU 120 as a transmission current detection signal.
Although the load voltage Vz corresponding to the load current Iz is shown as a constant voltage, the load current gently changes according to the environment temperature and the like.